In wireless communications, a physical channel between a transmitter and a receiver is formed by a radio link. In most cases, a plurality of different propagation paths exist between the transmitter and the receiver due to reflections in the environment, giving rise to a multipath channel with several resolvable components. Performance of a Code Division Multiple Access (CDMA) receiver may be improved using signal energy carried by several multipath signal components. The performance improvement is traditionally achieved by using a RAKE receiver in which each multipath signal component is assigned a despreader whose reference copy of a spreading code is delayed equally to a path delay of a corresponding multipath signal component. Outputs of the despreaders (i.e., RAKE fingers) are then coherently combined to produce a symbol estimate.
The RAKE receiver requires knowledge of multipath delays and values of a channel impulse response for all paths; combining weights are formulated as complex conjugates of net channel estimates for each of the delays. A RAKE+ receiver aims to improve a combined signal-to-interference ratio (SIR) by additionally scaling each combining weight by an impairment signal (i.e., noise plus interference) variance for each delay. An active finger selection (AFS) stage may follow the weight computation in order to exclude delays that, with high probability, do not correspond to a physical path.
The RAKE receiver combines signals corresponding to different path positions based on an assumption that individual-finger impairment signals are uncorrelated. This assumption is appropriate in predominantly non-dispersive channels; however, in strongly-dispersive environments, simple RAKE combining becomes sub-optimal, since significant correlation may appear between the individual fingers. The correlation is typically due to both a smearing effect of a receiver filter, which affects an originally “white” part of the impairment signal, and the fact that the multipath channel creates several copies of the same interference signal (i.e., a “colored” part of the signal).
The correlation is a redundancy in the received signal that can be utilized to further suppress the impairment component of the signal. One efficient receiver type developed to achieve this suppression is a generalized RAKE (GRAKE) receiver, as described in U.S. Pat. Nos. 6,363,104 and 6,714,585, which are both incorporated herein by reference. In dispersive environments, where the colored component of the interference dominates, the GRAKE receiver may serve to increase a post-combining SIR by several dB on average.
The GRAKE receiver typically requires one or more additional probing fingers, which additional fingers are placed on the pilot channel, in order to find a good solution. One or more additional combining fingers are also often placed on the data channels as well in order to extract most of the available interference suppression gains. Since the added fingers imply an increase in needed resources compared to a conventional RAKE combining of the same channel, a practical advanced receiver incorporating GRAKE typically only applies the GRAKE combining to traffic channels adapted to the channel reception quality for the particular user (e.g., power-controlled dedicated physical channel (DPCH) and high-speed downlink shared channel (HS-DSCH) with user-reported-SIR-dependent transport format and modulation scheme) and where better reception yields immediate concrete gains (e.g., lower average transmit power under transmit power control (TPC) or higher throughput in HSDPA). Common channels are not typically adjusted per-user by the network. Rather, the common channels, such as the broadcast channel (BCH), forward access channel (FACH), paging channel (PCH), random access channel (RACH), common packet channel (CPCH), and downlink shared channel (DSCH) in WCDMA, are transmitted with sufficient power to be received over a whole cell with a typical prior art WCDMA receiver. To save resources in a GRAKE receiver, the common channels, and possibly some other channels, are received using simple RAKE combining that requires fewer despreaders. For purposes of this application, channels so received are referred to as non-GRAKE (NGR) channels.
In the GRAKE receiver, despread values produced by RAKE fingers are combined to generate a decision statistic. Interference components of the different RAKE fingers are modeled as colored Gaussian noise to account for multipath dispersion and pulse shaping. The use of orthogonal spreading codes is accounted for when computing noise correlation between the fingers and when determining the noise powers on the RAKE fingers. The noise properties are used in a maximum-likelihood approach to determine combining weights. Finger placement is based on maximizing the signal-to-noise ratio of the decision statistic.
In contrast, in conventional RAKE reception, finger placement and combining weights corresponding to the channel impulse response of the signal of interest are used. In conventional RAKE receivers, the finger delays equal the channel delays and the weights are the channel coefficients. However, in the GRAKE receiver, the number of fingers and the combining weights are design parameters. The GRAKE receiver has the same general structure as the conventional RAKE receiver, but with different delays and weights. Unlike conventional RAKE receivers, GRAKE receivers benefit from using more fingers than the number of multipath signal components.